RFC 2071 Network Renumbering Overview January 19971. Introduction
The popularity of connecting to the global Internet over the course
of the past several years has spawned new problems; what most people
casually refer to as "growing pains" can be attributed to more basic
problems in understanding the requirements for Internet connectivity.
However, the reasons why organizations may need to renumber their
networks can greatly vary. We'll discuss these issues in some amount
of detail below. It is not within the intended scope of this
document to discuss renumbering methodologies, techniques, or tools.
2. Background
The ability for any network or interconnected devices, such as
desktop PCs or workstations, to obtain connectivity to any potential
destination in the global Internet is reliant upon the possession of
unique IP host addresses [1]. A duplicate host address that is being
used elsewhere in the Internet could best be described as
problematic, since the presence of duplicate addresses would cause
one of the destinations to be unreachable from some origins in the
Internet. It should be noted, however, that globally unique IP
addresses are not always necessary, and is dependent on the
connectivity requirements [2].
However, the recent popularity in obtaining Internet connectivity has
made these types of connectivity dependencies unpredictable, and
conventional wisdom in the Internet community dictates that the
various address allocation registries, such as the InterNIC, as well
as the ISP's, become more prudent in their address allocation
strategies. In that vein, the InterNIC has defined address
allocation policies [3] wherein the majority of address allocations
for end-user networks are accommodated by their upstream ISP, except
in cases where dual- or multihoming and very large blocks of
addresses are required. With this allocation policy becoming
standard current practice, it presents unique problems regarding the
portability of addresses from one provider to another.
As a practical matter, end users cannot assume they "own" address
allocations, if their intention is to be to have full connectivity to
the global Internet. Rather, end users will "borrow" part of the
address space of an upstream provider's allocation. The larger
provider block from which their space is suballocated will have been
assigned in a manner consistent with global Internet routing.
Not having "permanent" addresses does not mean users will not have
unique identifiers. Such identifiers are typically Domain Name System
(DNS) [4] names for endpoints such as servers and workstations.
Mechanisms such as the Dynamic Host Configuration Protocol (DHCP) [5]
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RFC 2071 Network Renumbering Overview January 1997
can help automate the assignment and maintenance of host names, as
well as the 'borrowed' addresses required for routing-level
connectivity.
The PIER Working Group is developing procedures and guidelines for
detailed renumbering of specific technologies, such as routers [6].
PIER WG documents are intended to suggest methods both for making
existing networks prepared for convenient renumbering, as well as for
operational transition to new addressing schemes.
Also, in many instances, organizations who have never connected to
the Internet, yet have been using arbitrary blocks of addresses since
their construction, have different and unique challenges.
3. Network Renumbering Defined
In the simplest of definitions, the exercise of renumbering a network
consists of changing the IP host addresses, and perhaps the network
mask, of each device within the network that has an address
associated with it. This activity may or may not consist of all
networks within a particular domain, such as FOO.EDU, or networks
which comprise an entire autonomous system.
Devices which may need to be renumbered, for example, are networked
PC's, workstations, printers, file servers, terminal servers, and
routers. Renumbering a network may involve changing host parameters
and configuration files which contain IP addresses, such as
configuration files which contain addresses of DNS and other servers,
addresses contained in SNMP [7] management stations, and addresses
configured in access control lists. While this is not an all-
inclusive list, the PIER working group is making efforts to compile
documentation to identify these devices in a more detailed fashion.
Network renumbering need not be sudden activity, either; in most
instances, an organization's upstream service provider(s) will allow
a grace period where both the "old" addresses and the "new" addresses
may be used in parallel.
4. Reasons for Renumbering
The following sections discuss particular reasons which may
precipitate network renumbering, and are not presented in any
particular order of precedence. They are grouped into reasons that
primarily reflect decisions made in the past, operational
requirements of the present, or plans for the future.
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RFC 2071 Network Renumbering Overview January 1997
Some of these requirements reflect evolution in the organization's
mission, such as a need to communicate with business partners, or to
work efficiently in a global Internet. Other requirements reflect
changes in network technologies.
4.1 Past
Many organizations implemented IP-based networks not for connectivity
to the Internet, but simply to make use of effective data
communications mechanisms. These organizations subsequently found
valid reasons to connect to other organizations or the Internet in
general, but found the address structures they chose incompatible
with overall Internet practice.
Other organizations connected early to the Internet, but did so at a
time when address space was not scarce. Yet other organizations
still have no requirement to connect to the Internet, but have legacy
addressing structures that do not scale to adequate size.
4.1.1 Initial addressing using non-unique addresses
As recently as two years ago, many organizations had no intention of
connecting to the Internet, and constructed their corporate or
organizational network(s) using unregistered, non-unique network
addresses. Obviously, as most problems evolve, these same
organizations determined that Internet connectivity had become a
valuable asset, and subsequently discovered that they could no longer
use the same unregistered, non-unique network addresses that were
previously deployed throughout their organization. Thus, the labor
of renumbering to valid network addresses is now upon them, as they
move to connect to the global Internet.
While obtaining valid, unique addresses is certainly required to
obtain full Internet connectivity in most circumstances, the number
of unique addresses required can be significantly reduced by the
implementation of Network Address Translation (NAT) devices [8] and
the use of private address space, as specified in [9]. NAT reduces
not only the number of required unique addresses, but also localizes
the changes required by renumbering.
It should also be noted that NAT technology may not always be a
viable option, depending upon scale of addressing, performance or
topological constraints.
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RFC 2071 Network Renumbering Overview January 19974.1.2 Legacy address allocation
There are also several instances where organizations were originally
allocated very large amounts of address space, such as traditional
"Class A" or "Class B" allocations, while the actual address
requirements are much less than the total amount of address space
originally allocated. In many cases, these organizations could
suffice with a smaller CIDR allocation, and utilize the allocated
address space in a more efficient manner. As allocation requirements
become more stringent, mechanisms to review how these organizations
are utilizing their address space could, quite possibly, result in a
request to return the original allocation to a particular registry
and renumber with a more appropriately sized address block.
4.1.3 Limitations of Bridged Internetworks
Bridging has a long and distinguished history in legacy networks. As
networks grow, however, traditional bridged networks reach
performance- and stability-related limits, including (but not limited
to) broadcast storms.
Early routers did not have the speed to handle the needs of some
large networks. Some organizations were literally not able to move
to routers until router forwarding performance improved to be
comparable to bridges. Now that routers are of comparable or
superior speed, and offer more robust features, replacing bridged
networks becomes reasonable.
IP addresses assigned to pure bridged networks tend not to be
subnetted, yet subnetting is a basic approach for router networks.
Introducing subnetting is a practical necessity in moving from
bridging to routing.
Special cases of bridging are realized in workgroup switching
systems, discussed below.
4.1.4 Limitations of Legacy Routing Systems
Other performance problems might come from routing mechanisms that
advertise excessive numbers of routing updates (e.g., RIP, IGRP).
Likewise, appropriate replacement protocols (e.g., OSPF, EIGRP, S-IS)
will work best with a structured addressing system that encourages
aggregation.
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RFC 2071 Network Renumbering Overview January 19974.1.5 Limitations of System Administration Methodologies
There can be operational limits to growth based on the difficulty of
adds, moves and changes. As enterprise networks grow, it may be
necessary to delegate portions of address assignment and maintenance.
If address space has been assigned randomly or inefficiently, it may
be difficult to delegate portions of the address space.
It is not unusual for organizational networks to grow sporadically,
obtaining an address prefix here and there, in a non-contiguous
fashion. Depending on the number of prefixes that an organization
acquires over time, it may become increasingly unmanageable or demand
higher levels of maintenance and administration when individual
prefixes are acquired in this way.
Reasonable IP address management may in general simplify continuing
system administration; a good numbering plan is also a good
renumbering plan. Renumbering may force a discipline into system
administration that will reduce long-term support costs.
It has been observed "...there is no way to renumber a network
without an inventory of the hosts (absent DHCP). On a large network
that needs a database, plus tools and staff to maintain the
database."[10] It can be argued that a detailed inventory of router
configurations is even more essential.
4.2 Present
Organizations now face needs to connect to the global Internet, or at
a minimum to other organizations through bilateral private links.
Certain new transmission technologies have tended to redefine the
basic notion of an IP subnet. An IP numbering plan needs to work
with these new ideas. Legacy bridged networks and leading-edge
workgroup switched networks may very well need changes in the
subnetting structure. Renumbering needs may also develop due to the
characteristics of new WAN technologies, especially nonbroadcast
multi-access (NBMA) services such as Frame-Relay and Asynchronous
Transfer Mode (ATM).
Increased use of telecommuting by mobile workers, and in small and
home offices, need on-demand WAN connectivity, using modems or ISDN.
Effective use of demand media often requires changes in numbering and
routing.
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RFC 2071 Network Renumbering Overview January 19974.2.1 Change in organizational structure or network topology
As companies grow, through mergers, acquisitions and reorganizations,
the need may arise for realignment and modification of the various
organizational network architectures. The connectivity of disparate
corporate networks present unique challenges in the realm of
renumbering, since one or more individual networks may have to be
blended into a much larger architecture consisting a different IP
address prefix altogether.
4.2.2 Inter-Enterprise Connectivity
Even if they do not connect to the general Internet, enterprises may
interconnect to other organizations which have independent numbering
systems. Such connectivity can be as simple as bilateral dedicated
circuits. If both enterprises use unregistered or private address
space, they run the risk of using duplicate addresses.
In such cases, one or both organizations may need to renumber into
different parts of the private address space, or obtain unique
registered addresses.
4.2.3 Change of Internet Service Provider
As mentioned previously in Section 2, it is increasingly becoming
current practice for organizations to have their IP addresses
allocated by their upstream ISP. Also, with the advent of Classless
Inter Domain Routing (CIDR) [11], and the considerable growth in the
size of the global Internet table, Internet Service Providers are
becoming more and more reluctant to allow customers to continue using
addresses which were allocated by the ISP, when the customer
terminates service and moves to another ISP. The prevailing reason
is that the ISP was previously issued a CIDR block of contiguous
address space, which can be announced to the remainder of the
Internet community as a single prefix. (A prefix is what is referred
to in classless terms as a contiguous block of IP addresses.) If a
non-customer advertises a specific component of the CIDR block, then
this adds an additional routing entry to the global Internet routing
table. This is what is commonly referred to as "punching holes" in a
CIDR block. Consequently, there are usually no routing anomalies in
doing this since a specific prefix is always preferred over an
aggregate route. However, if this practice were to happen on a large
scale, the growth of the global routing table would become much
larger, and perhaps too large for current backbone routers to
accommodate in an acceptable fashion with regards to performance of
recalculating routing information and sheer size of the routing table
itself. For obvious reasons, this practice is highly discouraged by
ISP's with CIDR blocks, and some ISP's are making this a contractual
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RFC 2071 Network Renumbering Overview January 1997
issue, so that customers understand that addresses allocated by the
ISP are non-portable.
It is noteworthy to mention that the likelihood of being forced to
renumber in this situation is inversely proportional to the size of
the customer's address space. For example, an organization with a
/16 allocation may be allowed to consider the address space
"portable", while an organization with multiple non-contiguous /24
allocations may not. While the scenarios may be vastly different in
scope, it becomes an issue to be decided at the discretion of the
initial allocating entity, and the ISP's involved; the major deciding
factor being whether or not the change will fragment an existing CIDR
block and whether it will significantly contribute to the overall
growth of the global Internet routing tables.
It should also be noted that (contrary to opinions sometimes voiced)
this form of renumbering is a technically necessary consequence of
changing ISP's, rather than a commercial or political mandate.
4.2.3 Internet Global Routing
Even large organizations, now connected to the Internet with
"portable" address space, may find their address allocation too
small. Current registry guidelines require that address space usage
be justified by an engineering plan. Older networks may not have
efficiently utilized existing address space, and may need to make
their existing structures more efficient before new address
allocations can be made.
4.2.4 Internal Use of LAN Switching
Introducing workgroup switches may introduce subtle renumbering
needs. Fundamentally, workgroup switches are specialized, high-
performance bridges, which make their main forwarding decisions based
on Layer 2 (MAC) address information. Even so, they rarely are
independent of Layer 3 (IP) address structure. Pure Layer 2
switching has a "flat" address space that will need to be renumbered
into a hierarchical, subnetted space consistent with routing.
Introducing single switches or stacks of switches may not have
significant impact on addressing, as long as it is understood that
each system of switches is a single broadcast domain. Each broadcast
domain should map to a single IP subnetwork.
Virtual LANs (VLANs) further extend the complexity of the role of
workgroup switches. It is generally true that moving an end station
from one switch port to another within the same VLAN will not cause
major changes in addressing. Many overview presentations of this
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RFC 2071 Network Renumbering Overview January 1997
technology do not make it clear that moving the same end station
between different VLANs will move the end station into another IP
subnet, requiring a significant address change.
Switches are commonly managed by SNMP applications. These network
management applications communicate with managed devices using IP.
Even if the switch does not do IP forwarding, it will itself need IP
addresses if it is to be managed. Also, if the clients and servers in
the workgroup are managed by SNMP, they will also require IP
addresses. The workgroup, therefore, will need to appear as one or
more IP subnetworks.
Increasingly, internetworking products are not purely Layer 2 or
Layer 3 devices. A workgroup switch product often includes a routing
function, so the numbering plan must support both flat Layer 2 and
hierarchical Layer 3 addressing.
4.2.4 Internal Use of NBMA Cloud Services
"Cloud" services such as frame relay often are more economical than
traditional services. At first glance, when converting existing
enterprise networks to NBMA, it might appear that the existing subnet
structure should be preserved, but this is often not the case.
Many organizations often began by treating the "cloud" as a single
subnet, but experience has shown it is often better to treat the
individual virtual circuits as separate subnets, which appear as
traditional point-to-point circuits. When the individual point-to-
point VCs become separate subnets, efficient address utilization
requires the use of long prefixes (i.e., 30 bit) for these subnets.
In practice, obtaining 30 bit prefixes means the logical network
should support variable length subnet masks (VLSM). VLSMs are the
primary method in which an assigned prefix can be subnetted
efficiently for different media types. This is accomplished by
establishing one or more prefix lengths for LAN media with more than
two hosts, and subdividing one or more of these shorter prefixes into
longer /30 prefixes that minimize address loss.
There are alternative ways to configure routing over NBMA, using
special mechanisms to exploit or simulate point-to-multipoint VCs.
These often have a significant performance impact, and may be less
reliable because a single routing point of failure is created.
Motivations for such alternatives tend to include:
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RFC 2071 Network Renumbering Overview January 1997
1. A desire not to use VLSM. This is often founded in fear
rather than technology.
2. Router implementation issues that limit the number of subnets
or interfaces a given router can support.
3. An inherently point-to-multipoint application (e.g., remote
hosts to a data center). In such cases, some of the
limitations are due to the dynamic routing protocol in use.
In such "hub-and-spoke" implementations, static routing can
be preferable from a performance and flexibility standpoint,
since it does not produce routing protocol chatter and is
unaffected by split horizon constraints (namely, the inability
to build an adjacency with a peer within the same IP
subnetwork).
4.2.5 Expansion of Dialup Services
Dialup services, especially public Internet access providers, are
experiencing explosive growth. This success represents a particular
drain on the available address space, especially with a commonly used
practice of assigning unique addresses to each customer.
In this case, individual users announce their address to the access
server using PPP's IP control protocol (IPCP) [12]. The server may
validate the proposed address against some type of user
identification, or simply make the address active in a subnet to
which the access server (or set of bridged access servers) belongs.
The preferred technique is to allocate dynamic addresses to the user
from a pool of addresses available to the access server.
4.2.6 Returning non-contiguous prefixes for an aggregate
In many instances, an organization can return their current, non-
contiguous prefix allocations for a contiguous block of address space
of equal or greater size, which can be accommodated with CIDR. Also,
many organizations have begun to deploy classless interior routing
protocols within their domains that make use of route summarization
and other optimized routing features, effectively reducing the total
number of routes being propagated within their internal network(s),
and making it much easier to administer and maintain.
Hierarchical routing protocols such as OSPF scale best when the
address assignment of a given network reflects the topology, and the
topology of the network can often be fluid. Given that the network is
fluid, even the best planned address assignment scheme, given time,
will diverge from the actual topology. While not required, some
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RFC 2071 Network Renumbering Overview January 1997
organization may choose to gain the benefit of both technical and
administrative scalability of their IGP by periodically renumbering
to have address assignments reflect the network topology. Patrick
Henry once said "the tree of liberty must from time to time be
watered with the blood of patriots." In the Internet, routing trees
of the best-planned networks need from time to time be watered with
at least the sweat of network administrators. Improving aggregation
is also highly encouraged to reduce the size of not only the global
Internet routing table, but also the size and scalability of interior
routing within the enterprise.
4.3 Future
Emerging new protocols will most definitely affect addressing plans
and numbering schemes.
4.3.1 Internal Use of Switched Virtual Circuit Services
Services such as ATM virtual circuits, switched frame relay, etc.,
present challenges not considered in the original IP design. The
basic IP decision in forwarding a packet is whether the destination
is local or remote, in relation to the source host's subnet. Address
resolution mechanisms are used to find the medium address of the
destination in the case of local destinations, or to find the medium
address of the router in the case of remote routers.
In these new services, there are cases where it is far more effective
to "cut-through" a new virtual circuit to the destination. If the
destination is on a different subnet than the source, the cut-through
typically is to the egress router that serves the destination subnet.
The advantage of cut-through in such a case is that it avoids the
latency of multiple router hops, and reduces load on "backbone"
routers. The cut-through decision is usually made by an entry router
that is aware of both the routed and switched environments.
This entry router communicates with a address resolution server using
the Next Hop Resolution Protocol (NHRP) [13]. This server maps the
destination network address to either a next-hop router (where cut-
through is not appropriate) or to an egress router reached over the
switched service. Obviously, the data base in such a server may be
affected by renumbering. Clients may have a hard-coded address of the
server, which again may need to change. While the NHRP protocol
specifications are still evolving at the time of this writing,
commercial implementations based on drafts of the protocol standard
are in use.
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RFC 2071 Network Renumbering Overview January 19974.3.2 Transitioning to IP version 6
Of course, when IPv6 [14] deployment is set in motion, and as
methodologies are developed to transition to IPv6, renumbering will
also be necessary, but perhaps not immediately mandatory. To aid in
the transition to IPv6, mechanisms to deploy dual- IPv4/IPv6 stacks
on network hosts should also become available. It is also envisioned
that Network Address Translation (NAT) devices will be developed to
assist in the IPv4 to IPv6 transition, or perhaps supplant the need
to renumber the majority of interior networks altogether, but that is
beyond the scope of this document. At the very least, DNS hosts will
need to be reconfigured to resolve new host names and addresses, and
routers will need to be reconfigured to advertise new prefixes.
IPv6 address allocation will be managed by the Internet Assigned
Numbers Authority (IANA) as set forth in [15].
5. Summary
As indicated by the Internet Architecture Board (IAB) in [16], the
task of renumbering networks is becoming more widespread and
commonplace. Although there are numerous reasons why an organization
would desire, or be required to renumber, there are equally as many
reasons why address allocation should be done with great care and
forethought at the onset, in order to minimize the impact that
renumbering would have on the organization. Even with the most
forethought and vision, however, an organization cannot foresee the
possibility for renumbering. The best advice, in this case, is to be
prepared, and get ready for renumbering.
6. Security Considerations
Although no obvious security issues are discussed in this document,
it stands to reason that renumbering certain devices can defeat
security systems designed and based on static IP host addresses.
Care should be exercised by the renumbering entity to ensure that all
security systems deployed with the network(s) which may need to be
renumbered be given special consideration and significant forethought
to provide continued functionality and adequate security.
7. Acknowledgments
Special acknowledgments to Yakov Rekhter [cisco Systems, Inc.], Tony
Bates [cisco Systems, Inc.] and Brian Carpenter [CERN] for their
contributions and editorial critique.
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